Anthracnose is destructive and infects leaves, pods and seeds especially during the period of heavy rainfall. The losses are mainly due to dropping of pods at young stage and discoloration and decay of some or all the seeds in the infected pods. In a harvest of affected crop, 86% pods have been found to be infected with 36% unmarketable seeds (Tucker 1927).

Symptoms

The typical symptoms are sunken spots on pods due to drying up and collapse of the cells in the center. The presence of pink sticky spore masses that develop during the moist weather have quite an ulcer like appearance. The main symptoms include spots on pods and leaves, blackening and shrinking of veins of infected leaves and their premature fall. The infected pods become distorted, abort and die. Affected young pods drop and all the seeds in the infected pods are discolored and decayed. Anthracnose spots appear as irregular, brown to grayish stem lesions containing dark, scattered acervuli. Severe infection leads to drying and death of infected branches. Minute spots with dark margins and gray centers scattered with acrvuli are also common on the leaves and pods of infected plants (Reddy et al. 1993).

The disease was first reported from Brazil in 1927 and subsequently from several other countries like Puerto Rico, India, USA, Venezuela and Zambia (Nene et al. 1996).

Biology and transmission.

The conidia are cylindrical, broadly elliptical or irregular with rounded ends and 12-17 ´ 3.5-7.2 m in size. The setae are numerous, and are curved, septate and 100 ´ 3.5 m in size. Appressoria apparently analogous to chlamydospores and were observed after 2 months on PDA and steamed pigeonpea pods. The pathogen has been associated with seeds of pigeonpea and is considered as the quarantine important pathogen (Nirula 1980).

Detection/indexing methods at ICRISAT

Anthracnose of pigeonpea:(C)acervuli on seed and (D)conidia. (photos: ICRISAT).

Over 380 hosts have been recorded in the USDA Systematic Mycology and Microbiology Fungus-Host Distribution Database. It is an opportunistic pathogen on numerous hosts causing leaf spots, rots and blights on many plant parts. It can also cause upper respiratory tract infections in AIDS patients and asthma in people with sensitivity.

Mycelium of the pathogen is usually light olive green to brown. Hyphae are dark brown, thick, septate and branched. Conidiophores are pale brown to olive brown, 25-60 ´ 3-3.5 µm in size and straight or flexuous. Individual conidiophores arise directly from substrate forming bushy heads consisting of 4-8 large catenate conidia chains. Secondary conidiophores are generally short and 1-celled. Conidia are pale brown to light brown, obclavate to obpyriform or ellipsoid, short conical beak at the tip, or sometimes beakless. Surface of conidia is smooth to verruculose, 20-63 ´ 9-18 µm in size. Conidia are septate with several (1-5) vertical and 3-8 transverse septa. The fungus sporulates well under warm and humid conditions (Kannaiyan and Nene 1977). Alternaria alternata has been found to be internally seedborne and the infection was detected in all parts of the seed (Girish et al. 2007).

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